Present invention relates to the polymerization of polyvinyl chloride, more particularly to suspension polymerization of polyvinyl chloride (PVC), and still more particularly to post polymerization treatment of the PVC suspension resin slurry which provides for more efficient removal of residual vinyl chloride monomer.
Homopolymers and copolymers of polyvinyl chloride have long been one of the largest and most versatile thermoplastic polymers, the vast majority of which has traditionally been manufactured by suspension polymerization techniques. In suspension polymerization, an intimate mixture of at least one polymerizable monomer and at least one suspending agent are suspended in aqueous medium and subjected to suitable polymerization conditions of time, temperature, and pressure. While there are countless literature and patent teachings of such processes, U.S. Pat. Nos. 2,833,754, 3,026,308, and 3,054,786 may be mentioned as three typical examples of such teachings.
In such techniques, the water and suspending agent(s) are first charged to the reactor, then the vinyl chloride monomer (and any comonomers) and the catalyst(s) are charged to the reactor, and the reaction is then permitted to continue until the desired degree of polymerization is achieved, usually as evidenced by a pressure drop in the reactor. At this point, the liquid medium is transferred to a blowdown tank or other suitable raw slurry storage tank (hereinafter "blowdown tank" for convenience), any gaseous vinyl chloride is collected, and the liquid phase is then subjected to a series of drying steps in which water is removed and the polymer is ultimately dried.
The most significant inherent undesirable property of polyvinyl chloride polymers is their susceptibility to degradation on exposure to heat and/or light. It is, of course, well known that such degradation, particularly heat degradation, is a function not only of the time and temperature of the immediate heat environment to which the PVC is subjected, but also a function of the "heat history" of the polymer, that is to say the cumulative effect of all the heat environments to which it has heretofore been subjected. Thus, where a particular mass of polyvinyl chloride polymer is subjected to even a brief period at moderately high temperature (e.g., in excess of about 200.degree. F.) though it may not evidence any discernable evidence of degradation, it will indeed have suffered adverse heat stability effects. If such a mass is subsequently subjected to additional heat conditions (e.g., during extrusion or other end use forming procedures), the cumulative heat history of the particular resin mass, as well as the immediate heat conditions to which it is then subjected, wil determine whether or not objectionable degradation will occur.
Certain essential process steps, of course, must be carried out in spite of this potential deleterious effect on the heat history of the polymer mass being treated. For example, the polymer must be dried, and all free water must be removed. While the last traces of water will indeed be removed by heat, the major portion of the water removal will be achieved by mechanical means, usually centrifugal or other mechanical filtration procedures.
With the increasing awareness of the potential dangers of vinyl chloride monomer (VCM), and the recognition that potentially significant concentrations of residual VCM can be entrapped in the polymer particle, new procedures have been introduced to promote recovery of unreacted VCM, and reduction of the residual VCM in the finished polymer particle. A wide variety of specific procedures have been suggested, and in many cases are in commercial use, to achieve these results.
In one such procedure, the PVC suspension resin slurry is stripped of residual vinyl chloride monomer in a colum using a combination of steam and pressure/vacuum control. The slurry from the blowdown tank is fed to top of the column, and flows downward over a series of trays to the bottom, while stream is added near the botom of the column and flows upward through the slurry, stripping it of VCM. The VCM-water vapor stream passes through the top of the column, the water is removed by condensation, and the recovered monomer is returned to the process. The stripped PVC slurry is fed from the bottom of the column to a cooling means and then to the drying and resin fin hing systems. See, for example, U.S. Ser. No. 592,119 filed July 1, 1975 and since abandoned in favor of C-I-P application Ser. No. 728,521, filed on Oct. 1, 1976.
Like drying of the resin, such a stripping procedure has an adverse effect on the heat history of the resin treated, but clearly it is now essential. In order to minimize the deleterious effect on heat history, the slurry being removed from the stripping column, which is usually at a temperature of from about 200.degree. to 240.degree. F., (about 93.degree. to 116.degree. C.) was fed to a cooling means where it was cooled as quickly as possible. One such cooling means is a conventional jacketed tank, using cool water to quickly lower the temperature of the slurry. On the other hand, one type of cooling means which has not heretofore been employed for such applications is a conventional heat exchanger, because of thhe tendency of the suspension resin particles to settle out, with the resultant blockage of the cooling means.